1. Field of Invention
The present invention relates to an airway adaptor for measurement of the concentration of carbon dioxide gas. Such an airway adaptor is attached to a respirator in order to measure the concentration of carbon dioxide gas in a respiratory gas.
2. Related Art
When respiration of a patient is artificially performed by using a respirator, an airway adaptor is attached to a flow path of the respirator, so as to measure the concentration of carbon dioxide gas in a respiratory gas. FIG. 4 is an external perspective view showing the configuration of an example of an airway adaptor of the prior art. Referring to FIG. 4, a center portion in the axial direction of a cylindrical flow tube 1 has a tubular shape having a rectangular section. Circular apertures 2 which allow infrared light to be transmitted therethrough are opened at opposed positions of side walls of the portion, respectively.
As shown in a section view of FIG. 5, a step portion 3 in which the inner side has a smaller diameter is concentrically formed in the inner periphery of each of the apertures 2. A transparent window 4 serving as a transparent film which is made of polyester or the like and formed into a disk-like shape is applied to the window via the step portion 3. An anti-fogging layer 5 is applied to or deposited on the inner face of the transparent window 4.
FIG. 6 is a section view showing the configuration of another example of a prior art method of fixing the transparent window 4 to the aperture 2, and corresponding to FIG. 5. Referring to FIG. 6, the outer periphery of the transparent window 4 is fixed to or heat-found on the inner periphery of the aperture 2 by the frame 6 with caulking the frame from the outside. In this case, the anti-fogging layer 5 on the inner surface of the transparent sheet 4 is substantially flush with the inner wall face of the flow tube 1.
In the prior art example shown in FIG. 5, the step is formed between inner wall of the flow tube 1 and the inner-surface of the anti-fogging layer 5, and hence the moisture content in a respiratory gas is condensed into waterdrops thereon. As shown in FIG. 7, the condensed waterdrop 7 gathers in the gravitational direction of the step portion. Therefore, the amount of infrared light which is transmitted through the window 2 is reduced, thereby producing a measurement error.
In the prior art example shown in FIG. 6, since the inner wall of the flow tube 1 is substantially flush with the inner face of the anti-fogging layer 5, there is no fear that water gathers around the inner periphery of the aperture 2 because there is no step portion as with the prior art example of FIGS. 5 and 7. When the airway adaptor is positioned such that transparent window 4 is arranged horizontally water condensed in a flow tube flows along the inner wall of the flow tube 1, however, water passes over the anti-fogging layer 5 as shown in FIG. 8, and hence there arises a fear that a measurement error may occur.